Conventionally, a radiation position detector which detects a position at which radiation is incident for a scintillator array including a plurality of scintillators is known. In this radiation position detector, Compton scattering may occur in radiation incident on a scintillator. When the Compton scattering occurs, the radiation may be simultaneously incident on two or more scintillators in the scintillator array. In this case, it is possible to increase the resolution of the radiation position detector by detecting the scintillator on which the radiation is first incident. For example, a radiation position detector described in Japanese Unexamined Patent Publication No. 7-49386 includes a scintillator array on which radiation is incident and an optical detector which photoelectrically converts a light emission phenomenon occurring in each scintillator of a scintillator array into a photoelectrically converted signal. In this radiation position detector, the levels of photoelectrically converted signal groups corresponding to a plurality of scintillators are compared with a plurality of different threshold values. A scintillator on which the radiation is incident is determined on the basis of features of signal pattern groups corresponding to the compared photoelectrically converted signal groups.